The present invention relates to a ferroelectric memory device, a method of driving the same, and a driver circuit.
A ferroelectric memory device including a ferroelectric capacitor as a storage device has characteristics such as an operation speed equal to that of a DRAM and nonvolatility similar to that of a flash memory, and is expected to be a memory device which may replace a conventional memory.
As the ferroelectric memory device, an active type ferroelectric memory device including a 1T/1C cell in which one transistor and one capacitor (ferroelectric) are disposed in each cell, or a 2T/2C cell in which a reference cell is further disposed in each cell, has been known. However, since the degree of integration of the 1T/1C cells and 2T/2C cells is limited, a memory device structure with a reduced size has been demanded taking an increase in the degree of integration in the future into consideration.
A ferroelectric material has a data retention function, and a memory operation can be achieved by using only a ferroelectric capacitor. Therefore, a ferroelectric memory device in which a memory cell is formed by only one ferroelectric capacitor (1C cell) has been proposed (see Japanese Patent Application Laid-open No. 9-116107 and published Japanese translation of PCT international publication for patent application No. 2001-515256).
However, when a positive select voltage is repeatedly applied to a selected memory cell, it is difficult to write data into the selected memory cell by applying a negative select voltage. This is because an imprint occurs in which the hysteresis characteristics of the ferroelectric memory cell shift on the negative electric field side in the direction in which the absolute value of the select voltage is increased. The imprint also occurs when the negative select voltage is repeatedly applied. In this case, the hysteresis characteristics of the ferroelectric memory cell shift on the positive electric field side in the direction in which the absolute value of the select voltage is increased. If an electric field in one direction is continuously and repeatedly applied to the selected memory cell, an error occurs in which the polarization of the ferroelectric capacitor is not reversed even if the select voltage with a large absolute value is applied.